How to determine the correct Sample Frequency for SPC Features

Romvill2002

Involved In Discussions
#41
Re: How do I determine a correct Sample Frequency for SPC features?

Hello Elsmar,

I need help. I have been reading through the web about IMR control chart and came across topics about inflated control limit. advised to multiply 3.144 to median but not sure where or how 3.144 was coming from.
Can someone enlighten me with this?

Thanks
 
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Bev D

Heretical Statistician
Staff member
Super Moderator
#42
Re: How do I determine a correct Sample Frequency for SPC features?

We need to be careful about the internet...there's a lot of misinformation and opinion out there.

"inflated control limits" and 3.144 multiplier and the median don't sound right in the context of an IMR chart.

can you point us to some of the references you have been reading to give us context?
 

Miner

Forum Moderator
Staff member
Admin
#44
Re: How do I determine a correct Sample Frequency for SPC features?

We need to be careful about the internet...there's a lot of misinformation and opinion out there.

"inflated control limits" and 3.144 multiplier and the median don't sound right in the context of an IMR chart.

can you point us to some of the references you have been reading to give us context?
I don't have time to look up the specifics, but I remember that there is a Median Range control chart (counterpart to the Xbar R chart). It was only used to eliminate the need to calculate the average (pre-computers). The control limits were 1.25x wider than the Xbar limits due to the difference in efficiency between the mean and the median as a measure of central tendency. However, this would only apply to subgrouped data, not individuals. The 3.144 might be the result of multiplying the 1.25 times the control limit constant.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#45
Re: How do I determine a correct Sample Frequency for SPC features?

I read the reference...the 'inflated limits' are described as being a result of one or more of the moving range values lying beyond the UCL of the moving ranges. The reference then recommends that the limits be adjusted narrower by multiplying 3.144*Median MOVING RANGE value...I suppose this could work (control charts aren't precise probability predictors) but it seems like a silly approach when the traditional method is much more straightforward. (As we would say in Indiana: it's like going around the barn to get the outhouse)

If you have any point or series of points that are 'out-of-control' in the initial limit calculations you have two choices: leave the limits as they are if they look fairly good (almost no process is 'in-control' when calculating initial limits; the limits aren't intended to be 'backwards' looking but forward looking and applied to future results point by point as they occur). OR if you know what caused the 'out-of-control' points you can simply remove them from the calculations.

I would suggest reading the 3 part series on "good limits from bad data" by Donald Wheeler and his articles on the IMR chart. (A simple search from these key words at QualityDigest.com will bring these up and they are free...

It might also be good to hear from Steve Prevette on this one.
 

Miner

Forum Moderator
Staff member
Admin
#46
Re: How do I determine a correct Sample Frequency for SPC features?

I started reading the reference. I did note that at the top of page 22, that these steps only apply to use of the IMR chart on attribute data. It also appears that the 3.144 multiplier is intended as a correction factor to tighten control limits that appear to be inflated per the guidelines on viewgraph 17 on page 23.
 

Romvill2002

Involved In Discussions
#47
Thanks for the very discussion, my problem with the multiplier is that if I establish it in our procedures(docs), Im afraid we might get audited by FDA and no explanation how we got the number(3.144). Or to be safe, just keep traditional way in using IMR control charts.
 

Miner

Forum Moderator
Staff member
Admin
#48
What is your data type? As I indicated above, the 3.144 multiplier only applies to attribute (count or proportion) data. If you have continuous data, it does not apply.
 

Proud Liberal

Quite Involved in Discussions
#49
Re: How do I determine a correct Sample frequency for SPC features?

To clarify: SPC (including Xbar, R charts) does NOT rely on the process being Normal or even close to Normal. (Simple subgrouping - measuring sequential pieces - does rely on a homogenous process stream. If the process stream is not homogenous, then rational subgrouping must be employed.)


Xbar, R / Xbar, S and I, MR charts will also detect a change in the variation not just a mean change...
So where do you stand on Bob Doering's approach. Although I understand his approach, I'm handcuffed with using QC-Calc (Xbar / R charts only). Please expound on how you would approach should be used for sub-grouping if your only option was Xbar and R charting (what would be a rational sub-group for precision machining?)
 
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